The present invention relates to masking films and, more specifically, to a masking film which removably adheres to rigid, relatively smooth-surfaced substrates under a variety of conditions without the need for corona treatment or an adhesive.
Masking films are used in numerous applications as a protective coating or covering for surfaces, particularly smooth surfaces, such as acrylics, glass, polished or painted metals, glazed ceramics, and other smooth, relatively rigid surfaces. The masking film is applied to the surface to be protected and acts as a physical barrier to prevent scratching, scuffing and marring of the surface. Protection provided by masking films is particularly useful while these surfaces are being printed, transported, or otherwise handled prior to use.
Traditionally, protection for smooth surfaces has been provided via corona treated films and/or adhesive coated masking paper. Corona treated films are films exposed to an electrostatic discharge to increase the adhesion level of the film. This is accomplished through the production of surface oxidation of the film via the electrostatic discharge, increasing the attraction between the nonpolar surface of the film and the polar surface of the material to be protected. Such corona treated films are typically non-embossed and rely on a very narrow window of corona treatment to facilitate enhanced adhesion. However, disadvantages exist with this technique. For example, where too little corona treatment occurs, the masking film will not adhere to the surface to be protected. Conversely, where too much corona treatment occurs, it is common to find the masking film laminating to itself and/or laminating completely to the surface to be protected, at best requiring additional time, effort and costs to completely unwind the masking film and/or remove the masking material from the protected surface, and, at worst, ruining the protected material for its intended end use. Additionally, since corona treated masking films have a relatively high surface coefficient of friction, rigid wrinkles commonly form in the masking film. Such wrinkles are difficult, if not impossible, to remove, thus precluding the film from adequately protecting the surface to be protected and/or permanently distorting the surface to be protected, again ruining it for its intended purpose. Finally, corona treated polyethylene films commonly have numerous large gels and carbon specks associated with them which can produce dimples in, or otherwise mar, the surface to be protected. Gels are defined as unmelted polyethylene particles which range from a barely visible size up to larger than a pencil point.
Disadvantages are also associated with masking films using an adhesive coated paper. For example, where a masking material requiring an adhesive coating is used, moisture from atmospheric humidity or elsewhere can permeate the masking material and loosen or completely separate the masking material from the substrate surface to be protected. The tendency for moisture to adversely affect the performance of this type of masking film is increased where heat is required to activate the adhesive coating. Additionally, even where the masking material remains firmly adhered to the surface to be protected until its removal is desired, such removal can require the use of a solvent to remove trace amounts of the adhesive coating. The adhesive residue left behind on the surface is of particular concern where the surface being protected is to be used in a context where sanitary conditions are desired, such as in food industry applications. The use of an adhesive coating also requires the additional steps of applying the adhesive coating to the preformed film, as well as the expense of using, activating and storing one or more adhesives to be used as a coating. Finally, where heat-activated adhesive coatings are used, the time and expense of providing the proper amount of heat to the process to facilitate proper adhesion further complicates the process.
Recent advances in masking film technology have produced improved masking films formed without corona treatment or the use of adhesive coatings, including one side smooth, one side matted (xe2x80x9cOSMxe2x80x9d) masking films. Such OSM films are more fully described in U.S. Pat. Nos. 4,895,760 and 5,100,709, both assigned to Tredegar Industries, Inc., Richmond, Va. These advanced masking films rely upon the tendency for smooth surfaces to adhere to each other to produce an adequate and constant level of adhesion without the need for corona treatment and the use of adhesive coatings. Additionally, the matted side of the OSM films prevents blocking and wrinkling of such films by preventing a measure of intimate contact between the surfaces. Importantly, these improved OSM films avoid the numerous problems associated with the use of corona treatment and adhesives and are stable over time, even when exposed to moisture and ultraviolet light.
Despite the advanced nature of the OSM films, however, it was discovered that the level of adhesion produced by these improved masking films can vary with temperature and other conditions associated with the production and use of such improved films. At times, such conditions can result in a masking film exhibiting either too much or not enough adhesion level for the desired application. In other applications, it can result in the need for heaters to raise the temperature of the sheet so that proper application and adequate adhesion level are achieved. Moreover, since the level of adhesion produced is primarily a function of the interaction between the smooth surface of the masking film and the smooth surface to be protected, the smoothness of the surface requiring protection is a significant factor. This factor can present difficulties, and masking films of this type are of limited utility, where the surface to be protected is not particularly smooth.
Methods for producing such prior art films are relatively rigid and do not offer flexibility in the recipe for such films, thus producing films incapable of producing a variety of adhesion levels under a variety of production conditions for a variety of applications.
Thus, there remains a need for a masking film capable of providing an adequate level of protection to merely relatively smooth surfaces by providing a functional, adjustable and controlled level of adhesion between the masking film and the surface to be protected without the use of corona treatment or an adhesive and their associated disadvantages and under a variety of production and application conditions, and a method for producing same.
In accordance with the present invention there is provided a masking film which adheres to and provides protection for a relatively smooth surface without the need for an adhesive layer or corona treatment. Additionally, the improved film is preferably of the one side matted type so that blocking and wrinkling of the film are substantially minimized, if not completely eliminated. Moreover, the adhesion produced is not as dependent upon the smoothness of the surface to be protected. Importantly, the level of adhesion produced by the improved OSM film is adjustable so as to accommodate a variety of production and application conditions. For example, the improved masking film of the present invention will provide a functional level of adhesion to uncoated polycarbonates, acrylics, polyvinyl chlorides and polyesters at room or ambient temperature. Accordingly, for virtually any given processing environment, including temperature and line equipment layout, and desired application, the improved masking film of the present invention can provide an adequate level of adhesion to the substrate of interest. The improved masking film of the present invention also remains removably attached to a substrate surface even after the application of post-production heat loading processes, including, but not limited to, thermoforming, drape-forming and heat-bending.
The improved masking film of the present invention comprises a film preferentially having a smooth side, a rough side and, optionally, one or more core layers interposed between the smooth side and the rough side. The monolayer is preferably extruded and the multiple layers are preferably coextruded. The smooth side comprises at least one layer of a thermoplastic film. In use, the smooth side is applied to the relatively smooth surface to be protected. The rough side is also comprised of at least one layer of a thermoplastic film. The rough side is preferably matte embossed, but can be roughened via any suitable means. The rough side prevents the film from contacting as much surface area of itself, or any other surface, preventing blocking and wrinkling of the film. At least one core layer may be interposed between the smooth side and the rough side of the improved masking film and, if present, is also comprised of a thermoplastic film. In the monolayer embodiment, the smooth side and rough side are opposing sides of the single layer of the film.
The level of adhesion produced between the smooth side of the masking film of the present invention and the surface to be protected is adjustable via the introduction of certain polymers and co-polymers associated with the smooth side of the film. The controlled combination of such polymers and co-polymers has the affect of adjusting the level of adhesion produced between the smooth side of the masking film and the surface to be protected by the masking film. The identity and quantities of these polymers are dictated by the conditions (e.g., temperature) under which the masking film will be applied and ultimately used. Thus, for example, the masking film of the present invention can be produced so as to provide a functional and controlled level of adhesion to acrylics at room temperature without subsequent laminating or welding during heat-loading processes, such as thermobending or thermoforming. The level of adhesion can be adjusted to provide adequate levels of adhesion with substrates at a higher temperature.
In other embodiments of the improved masking film of the present invention, a one, two or multilayered masking film is produced including the polymer and co-polymer additives capable of adjusting the level of adhesion produced by the film. These films can be blown or cast. Monolayer or coextrusion of multiple layers may be employed. Additionally, in the multilayered embodiment, the layer including the smooth side of the improved masking film of the present invention may be laminated to the layer including the rough side, if desired. The blending of polymer and co-polymer additives of the smooth side of the masking film is controlled to produce desired tackiness of the resulting masking film.
According to the method of the present invention, the improved masking film is produced by preselecting the one or more primary components comprising the first skin of the improved masking film of the present invention. It is the surface of this first skin layer which will ultimately intimately contact and adhere with the surface of the substrate to be protected. Once selected, the relative percentages of the one or more components is also predetermined so as to produce a functional and controlled level of adhesion force produced under a given set of the substrate""s production conditions and environment.
The remaining skin and core layers, if present, are formed of a thermoplastic. The skin and core layers are preferably coextruded to form the improved masking film of the present invention. Due to the preselection of the components and their relative amounts, the resulting masking film is tailored to perform in the given production environments under the given conditions.